When a great speed reduction ratio is required the worm gear speed reducer is one of the most frequently used. It has, however, some limitations: a low load capacity and excessive wear. The low load capacity and excessive wear are inherent to the particular geometry because the pitch cylinder of the worm has only one point of tangency with the pitch cylinder of the gear and, thus, it has very few teeth of the driven gear in contact with the drive worm.
During the two previous centuries many patents were granted referring to schemes to increase the load capacity of the worm gear reducers by increasing the contact surfaces. The most well-known schemes to achieve this purpose have been, on one hand, a concave configuration of the circumferential toothed surface of the driven gear so that it partially envelops the worm and, on the other hand, a drive worm with a concave silhouette that partially envelops the driven gear. Furthermore, there are double enveloping worm gears that incorporate simultaneously the two configurations described. These schemes have resulted in a tangible increase in the load capacity. However, the load capacity is still quite limited.
In 1897 patent U.S. Pat. No. 595,508 was granted to Wolander referring to the concept of a worm and chain reducer which resembles a worm gear, FIG. 1, but, instead of a driven gear, it drives a pair of chains with threaded links each mounted on a pair of identical sprockets. The worm drives the two chain transmissions. Since the worm is meshed with the threaded chains in the straight portion of the chain transmissions the surfaces in contact may be relatively large and, therefore, the load capacity may be greater than in the single or double enveloped schemes described in the previous paragraph. But, since the worm force has a moment arm with respect to the chainrods, jamming may result between the worm and the threaded links of either chain. In order to reduce this jamming moment arm, the threaded surface of the chain links envelops only a small fraction of the worm surface and this limits the load capacity. Furthermore, in order to minimize this limitation, the worm diameter is oversized, probably beyond that required to transmit the worm force and this implies an increase in the cost, in the volume and in the weight of the speed reducer. This applies as well to patent application JP2000-097293 by Yuuji and published in April 2000, even though in this case there is only one chain transmission, see FIG. 2.
In the schemes so far described the transmission of motion is carried out by sliding contact so the efficiency is low.
U.S. Pat. No. 418,328 granted to Willett and published on December 1889 refers to a mechanism to drive a boat by means of pedals and it includes a speed amplifier made up of a chain with idler rollers that drives a translation screw. In this case the efficiency is raised by the idler rollers. A similar scheme is used in U.S. Pat. No. 594,511 granted to Auble and published on November 1897, but in this case, it refers to the drive system of a land vehicle. Patents: U.S. Pat. No. 642,430 granted to Corcoran and published on January 1900; DE3305551 C2 granted to Reguzzi and published on September, 1990, FIG. 3; and U.S. Pat. No. 7,222,682, granted to Doering and DuPree published on May, 2007, also refer to chains with idler rollers that drive a translation screw. In all the patents mentioned in this paragraph the use of the idler rollers raises the efficiency, but their load capacity is limited because there is only one row of rollers per translation screw and because each roller makes contact with the translation screw only on a very short line segment.
Patents: U.S. Pat. No. 626,515, granted to Whitney and published on June, 1899; and U.S. Pat. No. 747,463, granted to Moore and published on December 1903, FIG. 4, refer to worm gear reducers which include idler rollers on the driven gear, which reduce friction, thus raising the efficiency. But, as has already been mentioned, this type of reducers have a limited load capacity due to their basic geometry and also because there are very few rollers which make contact with the drive worm.
In the last few decades there has been a great deal of interest in energy saving in general. In the particular case of worm gear reducers, patents have been granted, and continue being applied for, that involve concepts to make them more efficient by means of a plurality of balls that roll between the threads of the drive worm and the driven gear in a closed circuit, i.e., after rolling in the helical threads they enter a conduit that returns them to the helical path to start over. In this manner the sliding contact between the worm and the gear is substituted by a rolling contact which raises the efficiency of the reducer. These patents include the mutual partial envelopment of worm and gear to increase the load capacity but, as has already been pointed out, it is still limited by the restrictions imposed by the basic geometry of the worm and gear. Examples of this are: U.S. Pat. No. 4,023,433, granted to Schutz on May, 1977; patent application JP60-168936A by Tatsuo and Kazuharu, published on September, 1985; patent EP 0426461B1 granted to Ohtsuka and published on January, 1994; U.S. Pat. No. 7,051,610, granted to Stoianovici and Kavoussi on May, 2006; and patent application DE102013007462 A1 by Klaus published on November, 2014, FIG. 5.
Other patents which may be considered relevant prior art, as far as the roller screw used in the first embodiment of this invention is concerned, are U.S. Pat. No. 2,683,379, granted to Strandgren on July, 1954 and U.S. Pat. No. 8,082,818 granted to Sugitani on December, 2011. These patents do not refer to any speed reducer, they refer to the roller screw which is used in mechanical actuators and to activate the linear displacement of machine tool tables. The roller screw is driven by an internally threaded tube, but there are also inverted roller screws in which the tube is driven by the screw. The inverted roller screw refers basically to an assembly made up of a central sun screw, a plurality of threaded planetary rollers, and all of this is located inside an internally threaded tube. The threaded planetary rollers orbit around the central sun screw as they roll between it and the internally threaded tube driving the threaded tube in a straight line.
Patent application MX/a/2013/002354 and patent application US2014/0238162A1, now U.S. Pat. No. 9,234,570 B2 by Chicurel and Gutierrez refer to a screw and threaded chain speed reducer. The screw threads engage the threads of the single roller chain so that, as the screw turns, it drives the chain linearly. The force of the drive screw has a moment arm with respect to the chain rods, and this resulting moment may cause jamming between the drive screw and the “threaded bodies”. In contrast to this, in the speed reducer of the present invention, there is a threaded chain made up of a plurality of threaded bodies mounted on two parallel roller chains with extended rods. The extended rods serve as supports and also as pivots of the threaded bodies, and the centroid of the threaded surface of the threaded bodies is located in the plane of the extended rods, and therefore, the force that the screw exerts on the threaded chain is also located in said plane, thus in this arrangement there is no jamming moment.